The prior art includes directional microwave filter circuits, but does not include multiband microwave filters which can selectively transmit any discrete or combination of narrowband signals without resorting to the use of single pole multi-throw switches.
In a growing number of applications, it is necessary to be able to separate and identify many narrowband signals in a high density signal environment. Given the high processing speeds of currently available signal processors, microcomputers and minicomputers, such applications need multiband microwave filters which can switch frequency bands easily and quickly.
An obvious, but inadequate, design of a multiband microwave filter is one which connects a set of bandpass filters with two single pole multithrow switches. However, the maximum number of output ports for a multithrow switch is typically less than eight due to physical constraints, and the insertion loss of each switch is about 3 dB (decibels) at X-band frequencies. Consequently, any system with more than eight channels will require more than two multithrow switches--which will increase the signal loss accordingly.
It is therefore a primary object of the present invention to provide an multiband microwave filter which can selectively transmit any combination of a large number of frequency bands without resorting to the use of single pole multithrow switches.
Another object of the present invention is to provide a multiband microwave directional filter which can also be used as an allpass circuit.